1. Field of the Invention
This invention relates in general to earth boring drill bits, particularly to metallurgical treatments that increase the wear resistance, and hence the life of cutters and teeth exposed to abrasive wear.
2. Description of the Prior Art
The common prior art method for retarding the abrasive wear of teeth and cutters used in earth boring drill bits is the application of hardfacing to selected surfaces. A typical hardfacing used on teeth and cutters is a composite material consisting essentially of an alloy steel matrix in which is dispersed particles of sintered tungsten carbide. Prior art hardfacing application techniques include "sweat on" and "tube-application". To produce the sweat-on structure, an aqueous solution of sodium silicate is applied to the steel surface to be coated, tungsten carbide particles are brushed on or otherwise applied to the wet surface, and the water is allowed to evaporate. Heat is then applied to the surface via a welding torch until the steel is sufficiently fused to permit the particles to sink to a depth equal to approximately 75% of the granule diameter, after which the torch is moved on and the surface is allowed to cool.
In the tube type hardfacing, a combination of binder metal and tungsten carbide granules are applied simultaneously to a locally preheated surface. A welding torch is used to both heat the work area and to melt the tube of hardfacing. The tube consists of a hollow steel cylinder filled with a crushed or agglomerated tungsten carbide and sometimes with other materials, the ends of the tube being crimped or otherwise sealed to prevent the carbide filler from shaking out during storage and handling. The wall thickness and inside diameter of the tube are so related that the proportions of the two materials are known. A preferred composition contains about sixty weight-percent tungsten carbide to forty weight-percent binder.
The tube material can be a low carbon steel. Although such steels are relatively soft prior to use as a binder, they become abrasion resistant as the result of the alloying that occurs during application. Various other steels including alloy steels can be used as the binders in tube applications. When alloy steels are used, the alloying elements may either be incorporated in the carbon steel of the wall, or preferably, may be included as powders with particulate filler of tungsten carbide or the like. One such material thus added is a combination of ferromanganese and ferromolybdenum, the quantities being such that the over-all composition of the binder is about 1.0% manganese, 0.25% molybdenum, the balance being essentially low carbon steel.
Tube hardfacing can be made considerably thicker than sweat-on hardfacings without loss of integrity. As a result, tube hardfaced surfaces can be provided with an abrasion resistance which exceeds that of surfaces having sweat-on hardfacings.
In addition to providing increased abrasion resistance, hardfacings are typically applied to the teeth of rock bit cones to achieve a "self-sharpening" effect. Thus, in certain of the prior art bits, the hardfacing was applied on only one flank of the tooth. As the steel on the non-hardface flank wears or chips away, a sharp crest is maintained. In other of the prior art bits, a tube metal heavy deposit of tungsten carbide granules in an alloy steel matrix was applied on one part of the cone tooth. A sweat-on light application of hardfacing was applied to the tip of the other surface of the tooth. This structure also provided a "self sharpening" effect and maintained penetration rate. A third method of hardfacing to improve overall tooth wear resistance is known as "full flank" hardfacing. In this method, both flanks (and sometimes the inner and outer ends) of the tooth are hardfaced with the same tube material, the overall tooth wear resistance and life is improved, but the tooth loses the "self sharpening" effect generated by both of the above mentioned prior art methods and the wear tends to blunt the tooth rather than sharpening it, thus slowing the penetration of the bit. Despite the previously mentioned improvements, a need exists to improve the wear resistance of the tooth even further, without detracting from the self-sharpening effect.